Combination focusing and converging lens for multiple beam tubes



Nov. 18, 1958 R. E. BENWAY 2,861,208

COMBINATION FocusING AND coNvERGING LENS FOR MULTIPLE BEAM TUBES FiledMarch 1, 1955 2 Sheets-Sheet 1 Maxam/viz INVENTOR. Rasen-r EBENWBY MawNov. 18, 1958l R, E. BENWAY COMBINATION FOCUSING AND CONVERGING LENS 2sheets-sheet 2 FOR MULTIPLE BEAM TUBES Filed March l, 1955 INVENTOR.Rossm- E. BENWBY BY Robert E. Benway, Lancaster, Pa., assigner to RadioCorporation of America, a corporation of Delaware Application March 1,1955, Serial No. 491,468

4 Claims. (Cl. 313-70) "fhis invention relates to means for focusing andconverging a plurality of electron beams, and more particularly, to acombination focusing and converging lens for multiple beam tubes.

This invention is especially useful in cathode ray `viewingtubes forcolor television receivers. In such tubes, threeelectron beams aremodulated with three-color signals, respectively, and directed thruapertures in an aperture mask to a` three-color phosphor dot screen. Thethree electron beams, theapertures in the mask and the phosphor dots ofthe screen are so oriented that electrous of each beam strike only thephosphor dots of one color. it is necessary for the three electro-nbeams to be focused to small spots in the plane of the mask and screentarget. It is additionally necessary to converge the three beams so thatthey merge at the same'small spot in the `plane of the mask and screentarget.

It is au object of this invention to provide an improved and simplifiedelectrode structure for both focusing and converging a plurality ofelectron beams.

It is another object to provide an improved combina- States arent tionelectrostatic focusing and converging lens which l is relativelyinsensitive to power supply voltage Variations. It is a further objectto provide an improved multiple beam gun requiring one less directcurrent biasing potential than has previously been required.

it is a still further object to avoid distortion and arcing caused bycharges collecting on the glass mount beading by which the gunelectrodes are supported.

ln one aspect, the invention comprises a multiple beam gun for use in acolor kinesco-pe and including a cupshaped electrode mounted coaxiallyin the neck of the tube. The end of the cup-shaped electrode is providedwith symmetrically arranged apertures thru which the electron beams passon their way to theV target.` The inner-surface of the tube neckiscoated with a conductivesubstance which is biased with a considerablyhigher positive potential than is applied to the cup-shaped electrode.The dimensions of the cup-shaped electrode are so chosenthatftheresulting electric eld configuration causes a focusing of theindividualbeams as they pass thru the respectiveapertures in the end of thecup-shaped member, and also causes the beams toV converge to the samepoint at the target.

These and other objects and aspects of the invention will be apparent tothose skilled in the art from the following more detailed descriptiontaken in conjunction with the appended drawings, wherein: Y

Figure l is a broken sectional view of a color kinescope `constructedaccording to the teachings of thisinvention; f.

Figure 2 is a sectional View taken on Figure 1; p

Figure is an enlarged sectional View showingrthe combination focusingand `converging lens. forming a part ofthe color `kinescope of Figure 1;and p Figure 4' a. chart `which will be referred to in detheVY une 2 2of neck 10.

scribing how the relative dimensions of the cup-shaped and consists ofphosphor dots of three different` colors.

An aperture mask 14 is positioned in spaced relationshipV Y with thephosphor dot screen, each aperture in the mask 14 being in alignmentwith a cluster of `three phosphor dots of three diiferent colors on thescreen 1 The neck end of the envelope is provided with a base`equilateral triangle, a first grid assembly 17, a pre-g,

focusing second grid assembly 18, and a third grid assemf bly 19. Aconductive coating 20 on the inside of the` neck 10 of the tubeconstitutes a fourth or accelerating electrode. The usual deflectionyoke 9 surrounds the The third grid assembly 19 includes a cylindricalmember 21, an apertured end plate 22 thru which the beams enter from theleft as diverging beams, an apertured plate 23 positioned at anintermediate point in the cylinder 21,

and an apertured cup-shaped electrode 25. The apertures in the plate 23are slightly smaller than the size of to be equally spaced andequidistant from the longitudinal axis 28 of the tube. The respectiveapertures in all the grid assemblies and the respective cathode 16' tare in alignment. The grid assemblies are mechanically mounted ininsulated relationship by means of glass mountbeadings 24.

Referring to Figure 3, showing an enlarged portion of Figure l, thecup-shaped electrode 25 consists of a circular end Wall or plate 26 anda cylindrical wall 27. The cylindrical wall 27 has an axial length L anda diameter D. The apertures in the end plate 26 have centers spaced adistance g from the longitudinal axis 2S of the tube. p The aperturesare round and have a diameter a. erating electrode 20 consisting of aconductive coating ori the inner surface of the neck 10 has a diameterd.

rthe accelerating electrode 20 may, for example, be biased to apo-tential of 27,000 volts with respect to the p first grid 17, andthethird grid assembly 19 including cup-shaped electrode Z5 may be biasedto a potential of 3,000 volts relative to the first grid 17. Because ofthe potential difference between the accelerating electrode 20 and thecup-shaped electrode 25, and because of the physical geometry of theelectrodes, a complex electric field configuration is` created whichmay-Ibo illustrated by the dashed equi-potential lines in Figure 3. Theelectric force exerted upon electrons passingn thru the field is exertedat right angles with the equil potential lines.

It rwill be noted that some of the equi-potential lines in Figure 3bulgeinto and thru the apertures in thetcirthe tube. In order to preventfocusing distortion, it is necessary that the equi-potential lines inthe apertures be Patented Nov. 18, 1958 l A phosphor dot screen The gunincludes.l

The accel- V.portions vof end`26vnear the apertures therein.

perfectly symmetrical; Ifithe-lines are to some extent lopsided, thebeambecomes distorted as it goes towards the target so that it ceases tovhave a circular cross-section and does not focus to a small'spotvat thetarget. Symmetry -of' the equi-ipotentialllines inthe apertures isaccomplished by proportioning the cup-shaped electrodel 25fs'oV that theequi-potential linesvk adjacent to thel end plate 26 inlthe region ofythe apertures Aare substantially paralleli tothe endv plate 26. This isaccomplished by making length L of thecylindrical'wall 27 smallcomparedn withthe diameter D. Thev lengthv L should be less thanone-halfthe diameter D.

large relativeltothe'diameter Diso that the equi-potentiallinesfhave;suicientlcurvatureto cause the three beamsv to converge atthe same point at the target.

It is thus far apparent that the cup-shaped electrode 25 must-haveacylindrical'wall 27 of length Lsuiciently small relative to thediameter D -to'provide undistorted focusing, andlthe length-L must-besutiiciently large rela-1 tiveV to the diameter D to. provide isufficient convergence. Byproperly proportioningthecup-shaped electrode25, bothfocus`` and convergence can be obtained at the target.Asanlexample ofa tube actually constructed andsuccessfllyoperatedgthefcup-shaped electrode 25 had f Vthe dimensionsshown onFigure 3 of the drawings.v

The focusingl'enses-formed in the apertures in the end plate 26` shouldbe? shielded fromV electric fields which may be set-*up byotherrelectrodesin the-tube. This shielding is performed bythecylindrical member 21 forming apartof the third gridassembly 19. AllpartsV of the third gridv assembly 19, including the cup-shapedelectrode 25, are in` electrical contact with each other and arevbiasedto the same potential.- rPhe cylindrical member 21 provides a field-freepath for thebeamsso that no disturbingl electric'iield can' 'exist on.theA left- Yhand 'side of the end plate-26'and symmetricallenses areestablished inthe apertures intendA 26.'--

rlhef-proper length Lof-"the cylindrical wall 27relativeto'-tl1`e1diameter D is influenced by the-fdiametera andi thefspacing g1 of the' apertures in' the end plate 26.

The* larger? thendiairneterl av and* spacing g; the' shorter should'bethe length L and/ orthelargershould be they diameter D', inl-order toavoid focus distortion due to lack of-parallelisrn betweenequi-potential lines and the vFocus distortionl can alsol result frominsufficient Vspacing be tween'1v thezapertures.) The'ratio-ot theaperture spacing gto `thezaperture diarrieter a should be greater than0.9

inorder tof prevent interaction between the three focus-- inglens'es-in'thefapertures in end`f'265i If theV apertures v areftoo`c1os`etogether, the field in one Yaperture distorts theeldsinthe otherapertures. 'Y

'Achart'of Figure-4"illustrates how the proper length L.for"thecylindrical'wall 27y was experimentally determined; Starting with acup-shaped' electrodehaving a trom-Figurewi"that'whenf'the` lengtlrl:of` thel cylindrica static tubes.- It has beenfoundthat a yone, percentpowe f charged and'thereby-` distorting the focusing andconvrg khaving acylindrical wall' and Van 'endvwallipr'ovidedw 4- v the degree ofconvergence was noted. It will be seen i wall 27 was greater than0.63`inch, thel threebeams were. over-converged so that they crossedbefore reaching the i A target. Similarly, when the length L was lessthan 0.63 the beams were under-converged. With a length L Tof* Y 0.63inch both focus and'V convergence was achieved. 3 In practicing theinvention, the apertures in the end, plate 26v shouldbe designed soYthat' the ratio of g to' is greater than 0.9, where g is the distanceofthe aper ture centers fromthe axis 2S, and ais thezape'rture dianeter. In this way, interaction between` the focusing lenses in theapertures is avoided. With givenA values of g and a, the dimensions Land D of the cup-shaped elec trode 25 must be Selected'so that theequi-potential liner adjacent to the end plate 26 in the region of theapertures are substantially parallel to the end plate 26. Under thiscondition, the equi-potential lines extending into the apertures are.Asubstantiallysymmetrical; with. the; aper turesso that there is nofocust distortion. This ,is-accomg plished, by making L relatively smalllcomparedw-itli; D, and more speciiically, by making L less'thamone half.D.- f It willbe apparentrto those skilled in the art that objects ofthe invention can be achieved with structuren` havingdifferentdimensions than those shownin Figur 3. For example,xif thevspacing gofthe apertures i greater than .that shownit will be necessaryto increasesY the diameter ID. and/ or reduce the length Lof the.cfushaped electrode. 25 in order tomaintain parallelism betweentheequi-potential linesadjacent to the-end plat 26 in the region of theapertures. Similarly, if the spa ing g of the-apertures is less Vthanthat shown.inve changes should be made inthe dimensions Dfand L. A thesame time, the diameter D must be kept Ismall enough; and/ or thelengthL must be' kept large enough tofslimulr taneously provide kkthenecessary degree of convergence According to this invention,.asinglegridassembly Y1.9 in combination with the acceleratingelectrodej20serv to. both focus and converge the beams. Heretofore, ,ihas been necessary to provide one gridvassemblyvfor focusing and aseparate Vgrid assembly forco'nverging. It has beenznecessary to applydierent vbiasing potentiay to the two grid assemblies. Accordingto thisinvention a .single grid. assembly is employed forbothz'purposes and.only one source of, biasing jpgotentialisv used: co pared with the twopreviously necessary. rsafresujl there are-savings in the power. supplyas.l well as savings in the` number, of electrodes inthetubeandrareducti in.k the number of, bias potentials whichY must. beiV applithru.the'prongs;inthebase-.of the.tube.- t y -In .ahtube constructedaccordingvto thistinvention, the convergence is muchlessaiected lbypower supply yvolt-.xV age variations than hasbeen the case with priorelectro supply voltage change results in. onlynarfour mil:(0.0 inch)separation of eachI spot from the axis 2 8-'at target of the tube.Thisconvergence sensitivity isab'o one-half the Abestwhich,rhastuheretofore, been obtaineg; with an,y electrostaticconverging `lens.. "l

Aifu'rther advantage of the gun. structure sof` this ini/en tion is thatthev glass beading 24 used to. mountthe c1652- trodesoftthe. gun in'insulated Yrelaticinshipfis s o arran thatthere. is.Y noY possibilityvof the beading becom ing fields. j

According to Y.the teachingsbf this, invention there provided animproved multiple beam cathode, ray'. Y

including a simplified third grid assembly which sim taneouslyfocusesand converges the beams. 1

, Whatis claimed is: v l

1. In a triple, beam. gun., means to,Y focus andconver the: electron,beams -comprisingya cup-shaped electr' three circularY aperturesequally."spaced.-'fromeach `o and from the center of said end wall, acoaxial cylindrical accelerating electrode of larger diameter than saidcupshaped electrode and surrounding at least the open end of saidcup-shaped electrode, and means spacing and insulating said cup-shapedelectrode from said cylindrical accelerating electrode whereby saidaccelerating electrode may be maintained at a higher positive potentialthan said cup-shaped electrode, the ratio of the axial length of saidcup-shaped electrode to the diameter thereof being less than 0.5 toprovide undistorted focusing and to provide convergence.

2. -In a multiple beam gun, means to focus and converge the electronbeams comprising, a cylindrical accelerating electrode, a cup-shapedelectrode mounted coaxially with said accelerating electrode, saidcup-shaped electrode having a circular end wall provided with circularapertures for said beams and a cylindrical wall with the open edgethereof extending at least partially within said accelerating electrode,and means spacing and insulating said cup-shaped electrode from saidcylindrical accelerating electrode whereby a. potential difference maybe maintained between said electrodes, the axial length of saidcylindrical wall being less than one-half the diameter thereof.

3. In a multiple beam cathode ray tube having a tubular envelope, aplurality of beam-forming electrodes in one end of said envelope, atarget `at the other end of said envelope, and an accelerating electrodeyon the inner surface of said envelope, a combination focusing andconverging electrode positioned between said beam-forming electrodes andsaid target, said focusing and converging electrode comprising acylindrical member and a coaxial cup-shaped member fixed thereto andhaving an end wall closing said cylindrical member, said end wall ofsaid cup-shaped member having a plurality rof beam apertures, the openedge of said cup-shaped member being coaxially positioned within andspaced from said accelerating electrode, the axial length of thecylindrical walls of said cup-shaped member being less than one-half thediameter thereof.

4. In a multiple beam cathode ray tube having a tubular envelope, aplurality of beam-forming electrodes in one end of said envelope, atarget at the `other end of said envelope, and an accelerating electrodeon the inner surface of said envelope, a combination focusing andconverging electrode positioned between said beam-forming electrodes andsaid target, said focusing and converging electrode comprising acylindrical member and a coaxial cup-shaped member xed thereto having anend wall closing said cylindrical member, said end wall having aplurality of beam apertures symmetrically arranged around thelongitudinal axis of said focusing and converging electrode, the openedge Iof said cup-shaped member being coaxially positioned within #andspaced from said accelerating electrode, the axial length of thecylindrical walls of said cup-shaped member being less than one-half thediameter thereof, the spacing of the centers of said `apertures from thelongitudinal axis of the tube divided by the diameter of the aperturesbeing greater than 0.9.

References Cited in the file of this patent UNITED STATES PATENTS2,545,120 Swedlund Mar. 13, 1951 2,661,436 Van iOrmer Dec. 1, 19532,663,821 Law Dec. 22, 1953 2,690,517 Nicoll etal Sept. 28, 19542,717,323 Clay Sept. 6, 1955 2,729,759 Kratz et al. Ian. 3, 19562,761,990 Amdursky et al. Sept. 4, 1956

